Sharks are a group of elasmobranch cartilaginous fishes characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the division Selachii and are the sister group to the Batomorphi (rays and skates). Some sources extend the term "shark" as an informal category including extinct members of Chondrichthyes (cartilaginous fish) with a shark-like morphology, such as hybodonts. Shark-like chondrichthyans such as Cladoselache and Doliodus first appeared in the Devonian Period (419–359 million years), though some fossilized chondrichthyan-like scales are as old as the Late Ordovician (458–444 million years ago). The earliest confirmed modern sharks (Selachii) are known from the Early Jurassic around 200 million years ago, with the oldest known member being Agaleus, though records of true sharks may extend back as far as the Permian. Sharks range in size from the small dwarf lanternshark (Etmopterus perryi), a deep sea species that is only 17 centimetres (6.7 in) in length, to the whale shark (Rhincodon typus), the largest fish in the world, which reaches approximately 12 metres (40 ft) to 18.8 metres (62 ft) in length. They are found in all seas and are common to depths up to 2,000 metres (6,600 ft). They generally do not live in freshwater, although there are a few known exceptions, such as the bull shark and the river sharks, which can be found in both seawater and freshwater, and the Ganges shark, which lives only in freshwater. Sharks have a covering of placoid scales (denticles) that protects the skin from damage and parasites in addition to improving their fluid dynamics. They have numerous sets of replaceable teeth. Several shark species are apex predators, which are organisms that are at the top of their food chain with select examples including the bull shark, tiger shark, great white shark, mako sharks, thresher sharks, greenland sharks and hammerhead sharks. Some sharks are filter-feeding planktivores, such as the whale shark and basking shark, which are among the largest fish that ever lived. Sharks are caught by humans for shark meat or shark fins. Many shark populations are threatened by human activities. Since 1970, shark populations have been reduced by 71%, mostly from overfishing and mutilating practice such as shark finning.
Etymology Until the 16th century, sharks were known to mariners as "sea dogs". This is still evidential in several species termed "dogfish", or the porbeagle. The etymology of the word shark is uncertain. The most likely etymology states that the original sense of the word was that of "predator, one who preys on others" from the Dutch schurk, meaning 'villain, scoundrel' (cf. card shark, loan shark, etc.), which was later applied to the fish due to its predatory behaviour. It has been argued to derive from the Yucatec Maya word xoc/xooc (pronounced [ʃoːk]), meaning 'shark'.
Evolutionary history
Fossil record
The oldest total-group chondrichthyans, known as acanthodians or "spiny sharks", appeared during the Early Silurian, around 439 million years ago. The oldest confirmed members of Elasmobranchii sensu lato (the group containing all cartilaginous fish more closely related to modern sharks and rays than to chimaeras) appeared during the Devonian. Anachronistidae, the oldest probable representatives of Euselachii, the group containing modern sharks (Selachii) and rays (Batomorphi) to the exclusion of most extinct elasmobranch groups, date to the Carboniferous. Selachii and Batomorphi are suggested by some to have diverged during the Triassic. Fossils of the earliest true sharks may have appeared during the Permian, based on remains of "synechodontiforms" found in the Early Permian of Russia, but if remains of "synechodontiforms" from the Permian and Triassic are true sharks, they only had low diversity. Modern shark orders first appeared during the Early Jurassic, and during the Jurassic true sharks underwent great diversification. Sharks largely replaced the hybodonts, which had previously been a dominant group of shark-like fish during the Triassic and Early Jurassic.
Taxonomy
Sharks belong to the division Selachii in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates; the Chondrichthyes also include Chimaeras. It was thought that the sharks form a polyphyletic group: some sharks are more closely related to rays than they are to some other sharks, but current molecular studies support monophyly of both groups of sharks and batoids. The division Selachii is divided into the superorders Galeomorphi (or Galea), and Squalomorphi (or Squalea). The galeomorphs are the Heterodontiformes, Orectolobiformes, Lamniformes, and Carcharhiniformes. Lamnoids and carcharhinoids are usually placed in one clade, but recent studies show that Lamnoids and orectoloboids are a clade. Some scientists now think that Heterodontoids may be squalomorphs. The squalomorphs are divided into Hexanchiformes and Squalomorpha. The former includes cow shark and frilled shark, though some authors propose that both families be moved to separate orders. The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid. It includes the Squatiniformes, and the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and the Batomorphi. There are more than 500 species of sharks split across thirteen orders, including several orders of sharks that have gone extinct:
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